This invention relates to rotation detectors, and more particularly to a rotation detector suitable for use in a digitl type speedometer used in a vehicle such as a motor car.
Description in detail will hereunder be given of an example of the digital type speedometers with reference to the drawings. In addition, same reference numerals are used throughout the figures to designate members or elements having the same or similar functions. FIG. 1 shows an arrangement of a digital type speedometer, in which designated at reference numeral 2 is a transmission of a motor car and the transmission 2 is provided therein with a rotary drive shaft 4 rotatable in association with a rotary drive shaft, not shown, of an engine. As shown in FIG. 2, a rotary member 6 formed thereon with slits 6A disposed at regular intervals in the circumferential direction thereof is fixed to the forward end portion of the rotary shaft 4. The peripheral edge portion of the rotary member 6 is inserted in a light path of a photo-electric transducer (A photo-interruptor is used as the photo-electric transducer in this embodiment) constituted by an emission diode 8 and a light receiving element 10 (such as a photo-transistor).
Furthermore, denoted at 14 is a signal processing circuit for receiving output signals from the photo-electric transducer 12 to calculate the vehicle speed, and a calculation output from the signal processing circuit 14 is digitally indicated by an indicator 16.
Description will hereunder be given of an example of a detecting circuit of a digital type speedometer of the prior art shown in FIG. 1 with reference to FIGS. 3 and 4.
This detecting circuit is constituted by a photo-interruptor 12 and a detecting portion in the signal processing circuit 14. FIG. 3 shows the arrangement of the detecting circuit, in which the battery 18 is parallelly connected thereto with a series circuit of an emission diode 8 constituting the photo-interruptor 12 and a bias resistor 20.
Further, the battery 18 is parallelly connected thereto with a series circuit of a photo-transistor 10 for receiving a light emitted from the emission diode 8 through the rotary member 6, not shown, and resistors 22 and 24.
Additionally, the resistors 22 and 24 are connected to the base of a transistor 26, the collector of which is connected to an anode of the battery 18 and an emitter of which is connected to the cathode of the battery 18, respectively.
In the abovedescribed embodiment, the emission diode 8 constituting the photo-interruptor 12 and the resistor 20 are biased into a favorable direction and in continuity.
On the other hand, the light emitted from the emission diode 8 is intermittently interrupted by the rotation of the rotary member 6 (Refer to FIGS. 1 and 2) interposed between the photo-transistor 10 and the emission diode 8, the interrupted light causes an electric current shown in FIG. 4(A) to flow from the emitter of the photo-transistor 10 to the base of the switching transistor 26 through the resistor 22 in accordance with the rotating condition of the rotary member 6. As a result, pulse train signals shown in FIG. 4(B) are outputted from the collector, i.e. the output terminal 30 of the switching transistor 26. These pulse train signals are received in an operation circuit in the signal processing circuit 14 (Refer to FIG. 1) to calculate the vehicle speed.
Here, when the slits 6A of the rotary member 6 shut by about half the light path between the emission diode 8 and the photo-transistor 10, the photo-transistor 10 for detecting the light emitted from the emission diode 8 operates in an active region thereof, whereby the output level of the photo-transistor 10 comes into a mean level between the logical "1" and "0".
In this state, when external noises, switching noises from a power source in the case of using a DC/DC converter as the power source, noises caused by mechanical vibrations of the rotary member and the like are mixed with the output signals from the photo-transistor 10, the output level is fluctuated being centered about the mean level of the output signals, thereby leading to such a disadvantage that, irrespective of that the vehicle speed (rotational number) is 0 km/h for example, a misdetection is effected to indicate an unusually high vehicle speed.